Xiao H. Wang

The discrimination of excess toxicity from baseline effect: Effect of bioconcentration

Toxic ratio TR is a valuable tool in the discrimination of excess toxicity from baseline effect. Although some authors realized that internal effect concentration or critical body residual (CBR) calculated from bioconcentration factor (BCF) should be used in the TR, the effect of BCF on the discrimination of excess toxicity from baseline effect has not been investigated. In this paper, 951 acute toxicity data to fish (LC50) and 1088 BCFs were used to investigate the relationship between TR and BCF. The results showed that some compounds identified as reactive compounds exhibit excess toxicity, but some do not. BCF is closely related to TR and can significantly affect the TR value. The real excess toxicity which is used to identify reactive chemicals from baseline should be based on the toxic ratio of internal effect concentrations, rather than on the ratio of external effect concentrations, TR. The use of LC50 alone to determine TR can result in errors in TR because toxicokinetics (as estimated by the BCF) are ignored. The foundation in the discrimination of excess toxicity from baseline effect is based on the linear relationship between log BCF and hydrophobicity expressed as log KOW. However, log BCF is not linearly related with log KOW for all the compounds. The BCFs with log KOW >7 or <0 are either overestimated or underestimated by the linear baseline BCF model. Parallel lines are observed from calculated log CBR values for baseline and less inert compounds. The log BCF values overestimated or underestimated by log KOW from the baseline BCF model can result in mis-prediction and mis-classification among baseline, less inert and reactive compounds.

Discrimination of excess toxicity from narcotic effect: Influence of species sensitivity and bioconcentration on the classification of modes of action

The toxicity data of 2624 chemicals to fish, Daphniamagna, Tetrahymenapyriformis and Vibriofischeri were used to investigate the effects of species sensitivity and bioconcentration on excess toxicity. The results showed that 47 chemical classes were identified as having the same modes of action (MOAs) to all four species, but more than half of the classes were identified as having different MOAs. Difference in chemical MOAs is one of the reasons resulting in the difference in toxic effect to these four species. Other important reasons are the difference in sensitivity and bioconcentration of species. Among the four species, V. fischeri has the most compounds identified as reactive MOA. This may be due to some compounds can be easily absorbed into the bacteria, react with the DNA or proteins, disrupt the normal function of the cell and exhibit significantly greater toxicity to the bacteria. On the other hand, the skin and lipid content of aqueous organisms can strongly inhibit the bio-uptake for some reactive compounds, resulting in a less toxic effect than expected. D. magna is the most sensitive species and T. pyriformis is the least sensitive species of the four species. For a comparison of interspecies toxicity, we need to use the same reference threshold of excess toxicity. However, some reactive compounds may be identified as baseline or less inert compounds for low sensitive species from the threshold developed from high sensitive species. The difference in the discrimination of excess toxicity to different species is not only because of the difference in MOAs for some compounds, but also due to the difference in sensitivity and bioconcentration.

Comparison of Toxicities to Vibrio fischeri and Fish Based on Discrimination of Excess Toxicity from Baseline Level

Investigations on the relationship of toxicities between species play an important role in the understanding of toxic mechanisms to environmental organisms. In this paper, the toxicity data of 949 chemicals to fish and 1470 chemicals to V. fischeri were used to investigate the modes of action (MOAs) between species. The results show that although there is a positive interspecies correlation, the relationship is poor. Analysis on the excess toxicity calculated from toxic ratios (TR) shows that many chemicals have close toxicities and share the same MOAs between the two species. Linear relationships between the toxicities and octanol/water partition coefficient (log KOW) for baseline and less inert compounds indicate that the internal critical concentrations (CBRs) approach a constant both to fish and V. fischeri for neutral hydrophobic compounds. These compounds share the same toxic mechanisms and bio-uptake processes between species. On the other hand, some hydrophilic compounds exhibit different toxic effects with greatly different log TR values between V. fischeri and fish species. These hydrophilic compounds were identified as reactive MOAs to V. fischeri, but not to fish. The interspecies correlation is improved by adding a hydrophobic descriptor into the correlation equation. This indicates that the differences in the toxic ratios between fish and V. fischeri for these hydrophilic compounds can be partly attributed to the differences of bioconcentration between the two species, rather than the differences of reactivity with the target macromolecules. These hydrophilic compounds may more easily pass through the cell membrane of V. fischeri than the gill and skin of fish, react with the target macromolecules and exhibit excess toxicity. The compounds with log KOW > 7 exhibiting very low toxicity (log TR < –1) to both species indicate that the bioconcentration potential of a chemical plays a very important role in the identification of excess toxicity and MOAs.

Investigation on baseline toxicity to rats based on aliphatic compounds and comparison with toxicity to fish: Effect of exposure routes on toxicity

The aim of this paper was to investigate baseline toxicity to rats and effect of exposure routes on toxicity in rats and fish. In this paper, 1588 industrial chemicals were selected to investigate baseline toxicity to rats. The results showed that rat toxicity varies around a constant for classified compounds or homologues. The toxic contributions of substituted functional groups have been calculated and alkanes were used as baseline toxicity. The toxic contributions, equal to toxic ratios (TR), show that small changes in chemical structure can result in different toxic effect in rat toxicity. However, this situation has not been observed in fish toxicity because the threshold of excess toxicity (e.g. log TR=1) was too high to distinguish differences in toxicity. Very close critical body residues (CBRs) calculated from percentage of absorption and bioconcentration factors indicate that most of aliphatic chemicals may share the same modes of toxic action between rat and fish species. The high estimation error of bioconcentration factor calculated from computer programs for some compounds suggests that classification of excess toxicity should be based on the CBRs, rather than the TR because the TR is closely related to the exposure routes.

Comparison of toxicity of class-based organic chemicals to algae and fish based on discrimination of excess toxicity from baseline level

Toxicity data to fish and algae were used to investigate excess toxicity between species. Results show that chemicals exhibiting excess toxicity to fish also show excess toxicity to algae for most of the compounds. This indicates that they share the same mode of action between species. Similar relationships between logKOW and toxicities to fish and algae for baseline and less inert compounds suggest that they have similar critical body residues in the two species. Differences in excess toxicity for some compounds suggest that there is a difference of physiological structure and metabolism between fish and algae. Some reactive compounds (e.g. polyamines) exhibit greater toxic effects for algae than those for fish because of relatively low bio-uptake potential of these hydrophilic compounds in fish as compared with that in algae. Esters exhibiting greater toxicity in fish than that in algae indicate that metabolism can affect the discrimination of excess toxicity from baseline level. Algae growth inhibition is a very good surrogate for fish lethality. This is not only because overall toxicity sensitivity to algae is greater than that to fish, but also the excess toxicity calculated from algal toxicity can better reflect reactivity of compounds with target molecules than fish toxicity.

Mitochondrial bioenergetics and locomotor activity are altered in zebrafish (Danio rerio) after exposure to the bipyridylium herbicide diquat.

Diquat is a non-selective bipyridylium herbicide which has replaced its sister compound paraquat, as paraquat is associated to an increased risk for Parkinsons disease. However, the propensity of diquat to propagate reactive oxygen species ensures that diquat remains an exposure risk in non-target organisms. In this study, zebrafish (Danio rerio) embryos were exposed to diquat (1, 10, 100μM) beginning at ∼6h post fertilization for up to 7days to learn more about the mechanisms underlying diquat toxicity during vertebrate development. Zebrafish embryos exposed to diquat for 96h did not show any significant mortality nor deformity compared to controls. Moreover, there was no difference in the timing of hatch, an indicator of stress, for fish exposed to diquat. To determine whether changes in mitochondrial bioenergetics occurred in early development as a response to diquat exposure, oxygen consumption rate was measured in whole embryos. Basal respiration and ATP production were decreased following a 24h diquat exposure at 100μM, suggesting that diquat negatively affects oxidative phosphorylation. We also assessed locomotor behavior as a sensitive endpoint for impaired activity and neurotoxicity. Seven day old (7 dpf) zebrafish treated with diquat at the highest doses tested (10-100μM) showed an increase (hyper-activity) in total distance travelled, velocity, movement cumulative duration, and overall activity compared to unexposed fish. Lastly, in 7d fish, we measured transcripts related to redox balance and apoptosis as diquat has been reported to induce oxidative stress and can affect mitochondrial bioenergetics. Larvae exposed to 10μM diquat showed higher transcript levels of catalase compared to control fish, implying that reactive oxygen species are produced following diquat exposure. Transcript levels of sod1, sod2, bcl2, bax and caspase 3 however did not vary in abundance among treatments with diquat. This study improves mechanistic understanding of diquat in fish at early stages of development and presents evidence that diquat disrupts mitochondrial bioenergetics and behavior.

Discrimination of excess toxicity from baseline level for ionizable compounds: Effect of pH.

The toxic effect can be affected by pH in water through affecting the degree of ionization of ionizable compounds. Wrong classification of mode of action can be made from the apparent toxicities. In this paper, the toxicity data of 61 compounds to Daphnia magna determined at three pH values were used to investigate the effect of pH on the discrimination of excess toxicity. The results show that the apparent toxicities are significantly less than the baseline level. Analysis on the effect of pH on bioconcentration factor (BCF) shows that the log BCF values are significantly over-estimated for the strongly ionizable compounds, leading to the apparent toxicities greatly less than the baseline toxicities and the toxic ratios greatly less than zero. A theoretical equation between the apparent toxicities and pH has been developed basing on the critical body residue (CBR). The apparent toxicities are non-linearly related to pH, but linearly to fraction of unionized form. The determined apparent toxicities are well fitted with the toxicities predicted by the equation. The toxicities in the unionized form calculated from the equation are close to, or greater than the baseline level for almost all the strongly ionizable compounds, which are very different from the apparent toxicities. The studied ionizable compounds can be either classified as baseline, less inert or reactive compounds in D. magna toxicity. Some ionizable compounds do not exhibit excess toxicity at a certain pH, due not to their poor reactivity with target molecules, but because of the ionization in water.

Relationship between acute and chronic toxicity for prevalent organic pollutants in Vibrio fischeri based upon chemical mode of action

Chemicals show diverse modes of action (MOAs) in aquatic organisms depending upon acute and chronic toxicity evaluations. Here, toxicity data for Vibrio fischeri involving 52 compounds for acute and chronic toxicity were used to determine the congruence of acute and chronic toxicity for assessing MOAs. Using toxic ratios, most of the compounds categorized into MOAs that included baseline, less inert or reactive compounds with acute toxicity were also categorized as baseline, less inert or reactive compounds with chronic toxicity. However, significantly different toxic effects were observed with acute and chronic toxicity for the reactive and specific-acting compounds. The acute-chronic toxic ratios were smaller and less variable for the baseline and less inert compounds, but were greater and more variable for the reactive and specific-acting compounds. Baseline and less inert compounds share same MOAs, but reactive and specific-acting compounds have different MOAs between acute and chronic toxicity. Bioconcentration processes cannot reach an equilibrium for highly hydrophilic and ionized compounds with short-term exposure, resulting in lower toxicity compared to long-term exposure. Pronounced differences for the antibiotics were not only due to the difference in bioconcentration, but also due to a predicted difference in MOAs during acute and chronic exposures.

Fluazinam impairs oxidative phosphorylation and induces hyper/hypo-activity in a dose specific manner in zebrafish larvae

Fluazinam is a pyridinamine fungicide that induces oxidative stress and mitochondrial damage in cells, and it has been reported to be neurotoxic. To characterize the biological effects of fluazinam, we assessed mitochondrial bioenergetics, dopamine system expression, and behavior of early life staged zebrafish (0.01 μM-0.5 μM). Fluazinam at environmentally-relevant levels did not induce sub-lethal effects in larvae, but at the LC50 (0.5 μM), fluazinam decreased basal and ATP-linked respiration significantly in embryos. As mitochondria are directly related to redox homeostasis and apoptosis, the expression of genes related to oxidative stress and apoptosis were measured. Superoxide dismutase 2 (sod2), heat stock protein 70 (hsp70), bcl2-associated X protein (bax), and caspase 9 (casp9) mRNA levels were up-regulated by 0.5 μM fluazinam. Taken together, there was evidence for mitochondrial dysfunction and oxidative damage at the highest concentration of fluazinam (0.5 μM) tested. As there are reports for fluazinam-induced neurotoxicity in dopamine synthesizing cells, transcriptional targets in the dopamine system were assessed in the zebrafish. Tyrosine hydroxylase 1 (th1) and dopamine receptor 2a (drd2a) mRNA levels were decreased by 0.5 μM fluazinam, suggesting that this fungicide may affect the dopaminergic system. To further assess the potential for fluazinam-mediated neuromodulation, the dark photokinesis response was assessed in larvae following exposure. Larvae exposed to 0.1 μM fluazinam showed hyperactivity, while larvae exposed to 0.2 and 0.3 μM showed hypo-activity. This study demonstrates that fluazinam disrupts mitochondrial bioenergetics in zebrafish, inducing an oxidative stress response, and aberrant behaviors in larvae that are dose dependent.